This invention relates to an antenna for operation at frequencies in excess of 200 MHz, and particularly but not exclusively to an antenna having helical elements on or adjacent the surface of a solid dielectric core.
Such an antenna is disclosed in our co-pending British Patent Applications Nos. 2292638A, 2309592A and 2310543A, the entire disclosures of which are incorporated in this present application-so as to form part of the subject matter of this application as first filed. The earlier applications disclose antennas each having one or two pairs of diametrically opposed helical antenna elements which are plated on a substantially cylindrical electrically insulative core of a material having a relative dielectric constant greater than 5, with the material of the core occupying the major part of the volume defined by the core outer surface. A feeder structure extends axially through the core, and a trap in the form of a conductive sleeve encircles part of the core and connects to the feeder at one end of the core. At the other end of the core the antenna elements are each connected to the feeder structure. Each of the antenna elements terminates on a rim of the sleeve, each following a respective longitudinally extending path.
Such antennas can be used for the reception of circularly polarised signals, including signals transmitted by satellites of the Global Positioning System (GPS) which are transmitted at 1575 MHz. The antennas also have applications in the field of portable telephones, e.g. cellular telephones operating in UHF telephone bands, as described in the above-mentioned published applications. The applicants have determined that, at certain frequencies of interest, the feeder structure within the ceramic core can exhibit its own resonance which, if close to the required frequency of the antenna, can decrease antenna efficiency.
To overcome this difficulty, the present invention provides an antenna in which the feeder structure is spaced from the material of the solid dielectric core. In particular, the feeder structure is a coaxial transmission line provided with an outer sheath of dielectric material having a relative dielectric constant which is much lower than that of the core. In this way, the electrical length of, for instance, the outer conductor of a coaxial feeder structure is altered by virtue of being spaced from the high dielectric material of the core so that its resonant frequency is shifted with respect to the required operating frequency of the antenna to avoid coupling with the required resonant mode, thereby to increase antenna efficiency. Providing the thickness of the sheath is relatively small compared with the radial thickness of the core, i.e. between the outer surface of the sheath and the outer surface of the core, the required resonance due to the antenna elements on or adjacent the outer surface of the core is comparatively unaffected.